Different types of units for sealing by ultrasound are in industrial use today. Swedish Patent Application No. SE 9300918-1 discloses an ultrasound sealing unit which has been particularly developed for use in filling machines of the type which fills liquid foods into packages of the single use disposable type. There is but limited space available in these filling machines for an ultrasound sealing unit, and so the described unit is of a type which has a total length of one half of a wave length. In the filling machine, a material web of a laminate packaging material is formed into a tube which is sealed by a longitudinal joint or seam seal and is filled with the intended contents. The filled material tube is transversely sealed and, in these transverse seals, the material tube is severed to form individual packaging containers. The described sealing unit is intended for the transverse joint seals, for which reason the sealing unit displays an elongate, narrow sealing surface whose width corresponds to the width of the transverse joint seal. In order to achieve the requisite length of the transverse seal, a number of the above described sealing units must be built together to form a composite ultrasound sealing equipment unit. The sealing unit displays reaction bodies surrounding the drive unit, a horn which carries the elongate, narrow sealing surface, and a centrally located nodal plane where the amplitude of the unit is zero and where the unit may be secured in the filling machine.
A sealing unit of the above-outlined type will have an extremely complex geometry. Although, in the centrally located nodal plane, there is an amplitude which is zero, and although this nodal plane is employed for securing the unit, there is a slight residual motion in this plane because of the oscillation of the unit. This motion may be described as an undulation in one plane. The ceramic plates in the drive unit withstand considerable compression loading, but are brittle and highly sensitive to any type of flexural and tensile loading, for which reason the residual motion in the nodal plane may cause the ceramic plates to crack.
When a number of the above-described units are combined together to form an ultrasound sealing equipment unit in a filling machine, and when several filling machines may be employed in the same premises, it is vitally important that the different sealing units operate at the same amplitude in order to ensure complete exchangeability of parts. The piezoelectric ceramic plates are difficult to obtain with uniform characteristics, for which reason different drive units may have minor variations in the amplitude generated.